The present invention relates to methods of producing optical micro-structures. In particular, the present invention relates to using x-ray lithography on thick films of SOG materials to form the micro-structures.
There is an increasing use of integrated photonic and optoelectronic systems and a concurrent need for micro-optical components such as wave guides, couplers, lenses, prisms and similar devices for use in such systems. Silica is an excellent optical material because of its broad transmission range, low thermal expansion, high radiation hardness, mechanical strength and resistance to chemical attack. It is the basic constituent of optical fibers used in communication. Several prior art methods of producing silica micro-optical components begin with the step of forming silica films on a substrate such as a silicon wafer. These include a variety of chemical vapor deposition (CVD) and plasma-assisted deposition techniques. However, all these methods require high temperatures and/or vacuum techniques, and are generally suitable only for thin films.
Another method of forming silica films is through the use of organosilicon-based sol-gel and spin-on glass materials as precursors to silica. Sol-gel methods generally consist of gelation of silicon containing solutions which can be applied to a surface and then converted to silica through heating. The main advantage of sol gel methods is that silica may be produced at temperatures far lower than required for conventional melting processes. One technique for evenly distributing a silicon-based sol-gel film over a substrate consists of applying the sol-gel and then spinning the substrate at speeds sufficient to spread the gel across the substrate, hence the term xe2x80x9cspin-on glassxe2x80x9d or xe2x80x9cSOG.xe2x80x9d SOG materials are usually spin-coated onto silicon wafers as thin films not exceeding 1 xcexcm in a single coat. Thicker prior art SOG films on silicon wafers are prone to fail by cracking. The development of cracks in thick SOG films can be attributed to shrinkage during drying. Cracking of the film occurs because as shrinkage takes place, the SOG continues to adhere to the substrate, resulting in a buildup of tensile stress within the film. The thicker the film, the greater the shrinkage; and the greater the shrinkage, the greater the tensile stress in the film. In thick films, the magnitude of this tensile stress is severe enough to cause failure of the film. Thicker spin-coated SOG films can still be fabricated by a multiple-coating procedure where each coated layer is annealed before the application of the next layer. However, multiple coating involves too many repetitions, is too time consuming, and has difficulty in obtaining films with a thickness greater than 10 um. For the purposes of the disclosure herein, a xe2x80x9cthickxe2x80x9d film will be a film over 25 um.
The same type of sol-gels used for SOG techniques (xe2x80x9cSOG materialsxe2x80x9d) may also be used for molding techniques. Silica micro-structures, including micro-optical components, have been achieved by sol-gel micro-molding. See Marzolin, et al. 1998 and Han 1998. Typically these molds will be formed through some type of lithography and then the SOG material placed in the mold. However, problems with component release are to be expected with any molding method. Moreover, such problems could be avoided altogether if a process to fabricate these structures without a mold could be developed.
An example of direct lithography on a SOG material was found in Li et al., 1995 and Coudray et al., 1997. These methods involved direct UV lithography on a sol-gel processed photosensitive hybrid organosilicate glass and was used to fabricate waveguides and optical couplers. This is one of the few instances where actual products have been fabricated by direct lithography on a silicon precursor sol-gel film. Nevertheless, UV radiation typically will only penetrate a SOG material to a depth of a few tens of microns. This is generally not sufficient to produce high aspect ratio (e.g. a aspect ratio over approximately 8-10) which is desirable in many mirco-optical devices. The aspect ratio is generally defined as the height to width ratio of the structure. It would be a significant advance in the art if high aspect ratio silica structures could be formed directly onto a SOG material through a lithography process. This would provide the distinct advantage of manufacturing high aspect ratio silica structures without the need of a molding process.
The present invention provides a lithographic method for producing high aspect ratio silica micro-structures. The method comprises providing a carrier substrate with a confinement boundary placed on the carrier substrate. The SOG material is then placed within the confinement boundary and soft baked at a temperature above its glass transition temperature. Next, a pattern of interest is formed on the soft baked SOG material by x-ray lithography. Then the SOG material is heated until it is substantially converted to a silica-like oxide. This method provides a technique for moldless manufacture of silica-like micro-structures.
The present invention also includes a method of producing a hardened organosilicon film. This method also includes providing a carrier substrate and a SOG material have approximately the same coefficient of thermal expansion. A confinement boundary having a height of at least 25 microns is positioned on the carrier substrate and the SOG material placed within the confinement boundary. The SOG material is then soft baked at a temperature above the glass transition temperature of the SOG material in order to obtain a hardened organosilicon film.